Rotating electric machines (motors and generators) that employ permanent magnets are used in applications that need a compact and lightweight machine design and high machine performance. In general, a rotor of a rotating electric machine which uses permanent magnets includes a rotor core formed into a cylindrical shape by stacking thin steel plates in an axial direction. Stacking the steel plates provided with holes to insert permanent magnets forms permanent magnet insertion slots in the rotor core, so that the permanent magnets are mounted in the slots. In addition, the rotor core is fixed to a shaft that serves as a rotating shaft, and forms one of the major elements constituting the rotor.
Placement forms of the permanent magnets with respect to the rotor core can be broadly classified into a surface magnet type and an embedded (i.e., interior) magnet type. The surface magnet type is the form where, as described in Patent Document 1, for example, the permanent magnets are attached at predetermined intervals to an outer circumferential surface of the rotor core. The rotor of the surface magnet type can effectively use magnetic fluxes of the permanent magnets because the fluxes do not become short-circuited inside the rotor core. However, since the permanent magnets are liable to crack or peel by reason of centrifugal force, the surface magnet type is not suitable for high-speed rotating applications. In addition, an increase in a temperature of the permanent magnets due to a loss of an eddy current is liable to reduce efficiency. To improve these drawbacks, Patent Document 1 gives a description that in addition to first permanent magnets arranged on the outer circumferential surface of the rotor core, second permanent magnets are arranged at an inner diameter side of the first permanent magnets.
In a motor of the embedded magnet type, as described in Patent Document 2, for example, the permanent magnets are included in closed holes, that is, closed slots, of the rotor core, so the cracking or peeling of the permanent magnets due to centrifugal force can be prevented. A section of this rotor core that forms a structural member equivalent to one pole is shown in FIG. 12. In addition to the prevention of damage to the permanent magnets, this rotor structure has an advantage that the loss of the eddy current can be reduced. Furthermore, as in the motor described in Patent Document 2, providing two slots in a V-shaped arrangement to form one pole and then embedding a permanent magnet in each of the slots enables reluctance torque to be effectively used in the interpolar rotor core and hence a compact and highly efficient motor to be implemented. Moreover, in the motor of Patent Document 2, the slots in the rotor core have a resilient mechanism, which integrates the permanent magnets and the rotor core and enhances rigidity of the rotor.
Patent Document 3 refers to another rotor of the embedded magnet type. This rotor is substantially the same as that of Patent Document 2 in that the rotor core includes two permanent magnets embedded in each of V-shaped slots. However, a magnetic pole section and yoke section of the rotor core, respectively positioned at an outer diameter side and inner diameter side of the rotor core across the V-shaped slots, are not connected to one another and a filler formed from a curable nonmagnetic substance is placed between the two permanent magnets in each slot to form the rotor. Additionally, the V-shaped slots are notched toward an outer circumference of the rotor core. In other words, the rotor core employs an open-slotted structure in which the slots are not internally closed holes of the rotor core and include a portion opened toward the outer circumference of the core.
Patent Document 4 refers to yet another rotor of the embedded magnet type. This rotor is substantially the same as that of Patent Document 2 or 3 in that the rotor core includes two permanent magnets embedded in each of V-shaped slots. The slots formed in the rotor core, however, are constructed by alternate stacking of two kinds of steel plates, namely one kind with such closed slots as described in Patent Document 2, and the other kind with such open slots as described in Patent Document 3, and the two kinds of slotted steel plates form the rotor core.